Organic Carbon Budget Research Articles

The humic substance analogue antraquinone-2, 6-disulfonate (AQDS) enhanced zero-valent iron based autotrophic denitrification: Performances and mechanisms

Zero-valent iron based autotrophic denitrification (ZVI-AD) has attracted increasing attentions in nitrate removal due to saving organic carbon budget in wastewater treatment, but limited by the low reaction speed, poor electron transfer efficiency as well as the compaction/blocking by iron hydrolysis products. Humic substances (HS) were promising to regulate iron cycle and accelerate electron transfer by serving as electron mediators. In this study, HS analogue, antraquinone-2, 6-disulfonate (AQDS), was added to enhance ZVI-AD process. Results showed that the dosage of AQDS led to a NO3−-N removal efficiency of 83.37 ± 3.98% within 96 h, which was 32.28 ± 1.25% higher than that in ZVI-AD system. The corrosion of ZVI and microbially nitrate reduction were both improved at the presence of AQDS. The addition of AQDS enriched the functional species, including autotrophic denitrobacteria namely Thauera and Hydrogenophaga, iron redox-related species namely Ferruginibacter and HS respiration related species namely Flavobacterium. The genes napA and napB related to electron transfer, nirK and nosZ related to the accumulation of intermediate products were also enriched by the addition of AQDS. AQDS addition boosted the electrons flowing to both abiotic and biotic nitrate reduction. Nitrate removal mechanism involved in ZVI-AQDS coupled system was proposed. This study provided an alternative strategy for improving ZVI-AD by HS.

Seasonal and interannual variability of the pelagic ecosystem and of the organic carbon budget in the Rhodes Gyre (eastern Mediterranean): influence of winter mixing

Abstract. The Rhodes Gyre is a cyclonic persistent feature of the general circulation of the Levantine Basin in the eastern Mediterranean Sea. Although it is located in the most oligotrophic basin of the Mediterranean Sea, it is a relatively high primary production area due to strong winter nutrient supply associated with the formation of Levantine Intermediate Water. In this study, a 3D coupled hydrodynamic–biogeochemical model (SYMPHONIE/Eco3M-S) was used to characterize the seasonal and interannual variability of the Rhodes Gyre's ecosystem and to estimate an annual organic carbon budget over the 2013–2020 period. Comparisons of model outputs with satellite data and compiled in situ data from cruises and Biogeochemical-Argo floats revealed the ability of the model to reconstruct the main seasonal and spatial biogeochemical dynamics of the Levantine Basin. The model results indicated that during the winter mixing period, phytoplankton first progressively grow sustained by nutrient supply. Then, short episodes of convection driven by heat loss and wind events, favoring nutrient injections, organic carbon export, and inducing light limitation on primary production, alternate with short episodes of phytoplankton growth. The estimate of the annual organic carbon budget indicated that the Rhodes Gyre is an autotrophic area, with a positive net community production in the upper layer (0–150 m) amounting to 31.2 ± 6.9 gCm-2yr-1. Net community production in the upper layer is almost balanced over the 7-year period by physical transfers, (1) via downward export (16.8 ± 6.2 gCm-2yr-1) and (2) through lateral transport towards the surrounding regions (14.1 ± 2.1 gCm-2yr-1). The intermediate layer (150–400 m) also appears to be a source of organic carbon for the surrounding Levantine Sea (7.5 ± 2.8 gCm-2yr-1) mostly through the subduction of Levantine Intermediate Water following winter mixing. The Rhodes Gyre shows high interannual variability with enhanced primary production, net community production, and exports during years marked by intense heat losses and deep mixed layers. However, annual primary production appears to be only partially driven by winter vertical mixing. Based on our results, we can speculate that future increase of temperature and stratification could strongly impact the carbon fluxes in this region.

Vertical imbalance in organic carbon budgets is indicative of a missing vertical transfer during a phytoplankton bloom near South Georgia (COMICS)

The biological carbon pump, driven principally by the surface production of sinking organic matter and its subsequent remineralization to carbon dioxide (CO2) in the deep ocean, maintains atmospheric CO2 concentrations around 200 ppm lower than they would be if the ocean were abiotic. One important driver of the magnitude of this effect is the depth to which organic matter sinks before it is remineralised, a parameter we have limited confidence in measuring given the difficulty involved in balancing sources and sinks in the ocean's interior. One solution to this imbalance might be a temporal offset in which organic carbon accumulates in the mesopelagic zone (100–1000 m depth) early in the productive season before it is consumed later. Here, we develop a novel accounting method to address non-steady state conditions by estimating fluxes of particulate organic matter into, and accumulation within, distinct vertical layers in the mesopelagic zone using high-resolution spatiotemporal vertical profiles. We apply this approach to a time series of measurements made during the declining phase of a large diatom bloom in a low-circulation region of the Southern Ocean downstream of South Georgia. Our data show that the major export event led to a significant accumulation of organic matter in the upper mesopelagic zone (100–200 m depth) which declined over the following weeks, implying that temporal offsets need to be considered when compiling budgets. However, even when accounting for this accumulation, a mismatch in the vertically resolved organic carbon budget remained, implying that there are likely widespread processes that we do not yet understand that redistribute material vertically within the mesopelagic zone.

The organic carbon budget of an oligotrophic temperate peatland lake

AbstractLakes play a key role in the global carbon cycle, transporting, processing and storing organic carbon (OC) along the land‐ocean aquatic continuum. There are, however, surprisingly few complete lake OC budgets, particularly for certain lake types and geographical areas. An OC budget for Lough Feeagh (Ireland), an oligotrophic, peatland lake in a temperate oceanic location, was estimated for one year using both direct measurements and elements calculated from literature. It was constructed as a simple mass balance model that constrained the key OC processing rates. The total OC input to the lake during 2017 was estimated to be 2544 t C, of which it was estimated that 51% was imported as dissolved OC (DOC) in surface water, 4% in ground water, 1% in rainwater, and 3% was fixed in the lake as net ecosystem production. The remaining 41% was carried into the lake as particulate OC (POC) in surface water. The total C exported was estimated to be 2892 t C, of which 46% and 11% were exported as DOC and POC in the surface water outflow, respectively, 26% was deposited as sediment and 17% was mineralized and emitted as CO2 to the atmosphere. The results highlight the substantial volume of OC turned over in the lake during the study period. Moreover, it emphasizes how lakes in temperate, humic systems, common on the western edge of Europe, are important to the processing and fate of OC mobilized from their catchments.

Assessment of the enhanced weathering potential of different silicate minerals to improve soil quality and sequester CO2

Enhanced weathering is a negative emission technology that involves the spread of crushed silicate minerals and rocks on land and water. When applied to agricultural soils, the resulting increase in soil pH and release of nutrients may co-benefit plant productivity. Silicate minerals and rocks differ in their enhanced weathering potential, i.e., their potential for both carbon dioxide (CO2) sequestration and soil quality improvements. However, studies comparing silicate minerals and rocks for this dual potential are lacking. Therefore, we compared the enhanced weathering potential of olivine (Mg2SiO4), basalt, wollastonite (CaSiO3), and two minerals that are novel in this context, anorthite (CaAl2Si2O8) and albite (NaAlSi3O8). A down-flow soil column experiment was designed allowing for measurements on soils and leachate, and calculations of organic and inorganic carbon budgets. Our results showed comparatively high CO2 capture by enhanced wollastonite and olivine weathering. Furthermore, CO2 capture per m2 specific surface area indicated potential for enhanced anorthite and albite weathering. Calculated carbon budgets showed that most treatments produced net CO2 emissions from soils, likely related to the short duration of this experiment. All silicates generally improved soil quality, with soil nickel contents remaining below contamination limits. However, nickel concentrations in leachates from olivine-amended soils exceeded the groundwater threshold value, stressing the importance of monitoring nickel leaching. We found a relatively high enhanced weathering potential for wollastonite, while the potential for olivine may be constrained by nickel leaching. The promising results for anorthite and albite indicate the need to further quantify their enhanced weathering potential.

Effects of Hypoxia on Coupled Carbon and Iron Cycling Differ Between Weekly and Multiannual Timescales in Two Freshwater Reservoirs

AbstractFreshwater lakes and reservoirs play a disproportionate role in the global organic carbon (OC) budget, as active sites for carbon processing and burial. Associations between OC and iron (Fe) are hypothesized to contribute substantially to the stabilization of OC in sediment, but the magnitude of freshwater Fe‐OC complexation remains unresolved. Moreover, global declines in bottom‐water oxygen concentrations have the potential to alter OC and Fe cycles in multiple ways, and the net effects of low‐oxygen (hypoxic) conditions on OC and Fe are poorly characterized. Here, we measured the pool of Fe‐bound OC (Fe‐OC) in surficial sediments from two eutrophic reservoirs, and we paired whole‐ecosystem experiments with sediment incubations to determine the effects of hypoxia on OC and Fe cycling over multiple timescales. Our experiments demonstrated that short periods (2–4 weeks) of hypoxia can increase aqueous Fe and OC concentrations while decreasing OC and Fe‐OC in surficial sediment by 30%. However, exposure to seasonal hypoxia over multiple years was associated with a 57% increase in sediment OC and no change in sediment Fe‐OC. These results suggest that the large sediment Fe‐OC pool (∼30% of sediment OC in both reservoirs) contains both oxygen‐sensitive and oxygen‐insensitive fractions, and over multiannual timescales OC respiration rates may play a more important role in determining the effect of hypoxia on sediment OC than Fe‐OC dissociation. Consequently, we anticipate that global declines in oxygen concentrations will alter OC and Fe cycling, with the direction and magnitude of effects dependent upon the duration of hypoxia.

A RUpture‐Based detection method for the Active mesopeLagIc Zone (RUBALIZ): A crucial step toward rigorous carbon budget assessments

AbstractDetermining mesopelagic organic carbon budgets is essential to characterize the ocean's role as a carbon dioxide sink. This is because the biological processes observed in the mesopelagic zone are crucial for understanding the biological carbon pump. Yet, field assessments of carbon budgets are often unbalanced with the carbon demand exceeding its supply. This underlines either methodological issues in the budget calculations or incomplete knowledge of the mesopelagic carbon cycling with potentially missing sources. Carbon budgets are built by partitioning the ocean into vertical depth zones. Vertical boundaries are conventionally defined between 200 and 1000 m depth or using various thresholds. Such approaches lack consistent methodology preventing robust comparison of mesopelagic carbon budget from region to region. Here, using a statistical rupture detection method applied to conductivity–temperature–depth (CTD)‐cast variables (fluorescence, O2 concentration, potential temperature, salinity, and density), we aim to provide independent estimates of mesopelagic boundaries. We demonstrate that the so‐determined upper boundary is highly correlated with the knee points of the particulate organic carbon (POC) fluxes estimated by a power law and that over 90% of the POC flux attenuation occurs within our method boundaries. The identified zone therefore corresponds to the most active part of the conventional mesopelagic zone and we name it the “active mesopelagic zone” (AMZ). We find that the depths of the mesopelagic zone depend on the region considered. Our results demonstrate that the mesopelagic carbon budget discrepancy can vary up to four folds depending on the boundaries chosen and hence provide novel grounds to reassess existing and future mesopelagic carbon budgets.

Reviews and syntheses: VOC emissions from soil cover in boreal and temperate natural ecosystems of the Northern Hemisphere

Abstract. Plant litter decomposition is a biogeochemical process underlying the carbon cycle in terrestrial ecosystems and between the biosphere and the atmosphere. For the latter, it serves as one of the most important sources of not only carbon dioxide but also volatile organic compounds (VOCs), which have not yet been taken into account in atmospheric models for various purposes and scales, from local to regional and global. This review owes its appearance to the growing interest in decaying leaf litter and living forest floor cover as a hitherto unaccounted for source of photochemically active components of the Earth's atmosphere. This interest is understandable if we take into account the size of this source: for terrestrial ecosystems, the global production of litter is 10 × 1016 g dry matter. The living vegetation cover of the soil on the forest floor, mainly comprising mosses and small shrubs, should also be regarded as a potentially significant source of atmospheric VOCs, as its total biomass may be comparable to or even exceed that of canopy foliage, which is considered the main source of these compounds. This implies a need to integrate these sources into biogenic VOC emission models, which in turn requires extensive research on these sources to understand the conditions and factors that influence VOC emissions. The decomposition of leaf litter, accompanied by the release of VOCs, is a very complex process that depends on a number of biological, chemical and physical environmental factors, but little information is currently available on the role each plays. Equally limited is information on the chemical composition and emission rates of VOCs from these sources. The review focuses on the main gaps in our knowledge of the sources of biogenic VOCs under the forest canopy, and we are confident that filling them will make a significant contribution to solving such an important task as closing the global organic carbon budget.

The influence of shellfish farming on sedimentary organic carbon mineralization: A case study in a coastal scallop farming area of Yantai, China

This study quantified the rate and relative contribution of each sedimentary organic carbon (OC) mineralization pathway in the Yantai coastal area. The results showed that scallop farming activities with raft-breeding facilities led to increased accumulation of OC and reactive Fe(III), which in turn promoted OC aerobic mineralization, Fe reduction, and competitively inhibited sulfate reduction. In the scallop farming area (SFA), O2 reduction, dissimilatory Fe reduction, and sulfate reduction contributed 32.17 %, 27.77 %, and 30.18 % of the total OC mineralization, 1.6 times, 1.2 times, and 0.6 times those of the non-farming area, respectively. Nevertheless, scallop harvesting and resuspension by water currents will increase the short-term risk of dissolved inorganic carbon accumulation in the water column. The OC budget showed that the Yantai coastal area exhibited more OC mineralization than storage, with only 5.0–7.2 % of the net settled OC being permanently buried in the sediment.

Physiological acclimatization in Hawaiian corals following a 22-month shift in baseline seawater temperature and pH

Climate change poses a major threat to coral reefs. We conducted an outdoor 22-month experiment to investigate if coral could not just survive, but also physiologically cope, with chronic ocean warming and acidification conditions expected later this century under the Paris Climate Agreement. We recorded survivorship and measured eleven phenotypic traits to evaluate the holobiont responses of Hawaiian coral: color, Symbiodiniaceae density, calcification, photosynthesis, respiration, total organic carbon flux, carbon budget, biomass, lipids, protein, and maximum Artemia capture rate. Survivorship was lowest in Montipora capitata and only some survivors were able to meet metabolic demand and physiologically cope with future ocean conditions. Most M. capitata survivors bleached through loss of chlorophyll pigments and simultaneously experienced increased respiration rates and negative carbon budgets due to a 236% increase in total organic carbon losses under combined future ocean conditions. Porites compressa and Porites lobata had the highest survivorship and coped well under future ocean conditions with positive calcification and increased biomass, maintenance of lipids, and the capacity to exceed their metabolic demand through photosynthesis and heterotrophy. Thus, our findings show that significant biological diversity within resilient corals like Porites, and some genotypes of sensitive species, will persist this century provided atmospheric carbon dioxide levels are controlled. Since Porites corals are ubiquitous throughout the world’s oceans and often major reef builders, the persistence of this resilient genus provides hope for future reef ecosystem function globally.

Integrating Perspectives on Dissolved Organic Carbon Removal and Whole‐Stream Metabolism

AbstractQuantifying organic carbon (OC) removal in streams is needed to integrate the functional role of inland waters into landscape carbon budgets. To illustrate how in‐stream OC removal measurements can be used to characterize ecosystem and landscape carbon fluxes, we compared two common methods: (a) Bioassays measuring water column dissolved organic carbon (DOC) uptake and (b) daily rates of whole‐stream metabolism and OC spiraling calculated from fluorescent dissolved organic matter, oxygen, and discharge measurements. We then assessed how OC removal rates from these two methods, measured in two low‐productivity heterotrophic streams, affected estimates of terrestrial OC loading and export using a mass balance model. OC mineralization velocities calculated from whole‐stream metabolism (0.06 ± 0.03 m d−1 (mean ± SD)) were greater than water column bioassay DOC uptake velocities (0.01 ± 0.01 m d−1), which resulted in higher in‐stream OC removal estimates (0.5%–15.2% and 0.02%–4.2% removal for whole‐stream metabolism and bioassays, respectively). Furthermore, the terrestrial OC inputs needed to sustain in‐stream OC concentrations differ among methods, with simulated inputs ranging from 79 to 1,300 or 3–350 g OC d−1 for whole‐stream metabolism or bioassays, respectively. We show how in‐stream OC removal can be used to quantify terrestrial‐aquatic linkages by estimating OC inputs needed to fuel whole‐stream metabolism in low‐productivity streams, and offer future directions to better link OC removal with whole‐ecosystem OC budgets. Without appropriate conversions to whole‐stream processes, bioassays systematically underestimate whole‐stream carbon cycling. By integrating whole‐stream metabolism with OC transport, we can better elucidate the role of running waters in landscape carbon budgets and the global carbon cycle.

Organic carbon mineralization pathways in the muddy sediments of the South Yellow Sea: Insights from steady-state modeling of porewater

In marine sediments, microbial mineralization of organic carbon (OC) proceeds through a cascade of pathways using different electron acceptors. Rates and relative importance of individual pathways can provide quantitative constraints on biogeochemical cycling of a number of elements, such as carbon, nitrogen, sulfur, iron, and manganese. However, such information is hitherto very scarce in the China marginal seas. To fill the gap, a multicomponent-coupled reaction-transport model CANDI was applied to quantify the rates and relative contributions of individual pathways of OC mineralization at three sites in the South Yellow Sea (SYS) sediments, based on which OC budgets were constructed. Model results at steady state generally well-reproduced the profiles of measured porewater solutes. The fast degrading OC pools G1 become exhausted over the upper 4 cm at the studied sites with the degradation rate constants k1 ranging between 0.6 and 1.65 yr−1, while the slow degrading OC pool G2 remains almost undegradable on the timescales of the studied depth (∼25 cm) due to the low degradation rate constants k2 (10−5‒10−4 yr−1) The depth-integrated rates of OC mineralization at the studied sites are in the range of 5.1–18.3 mmol C m−2 d−1, and aerobic respiration stands out as the dominant pathway, with its average contribution of 62%, followed by sulfate reduction (22.6%), dissimilatory manganese/iron reduction (8.4%), and denitrification (6.1%). Methanogenesis is a negligible process of OC mineralization within the studied depths. The contributions of aerobic respiration in the SYS sediments are much higher than the global average for marginal seas, but the contributions of sulfate reduction and iron dissimilatory reduction are much lower than their global averages, which could be ascribed to the low availability of degradable OC for anaerobic respiration. Budget of OC indicates that the mineralization and burial of OC in the muddy sediments account for 7.3 and 1.7%, respectively, of the total OC supplied to the sea, with the net OC accumulation efficiency of ∼19%.

Monitoring of organic carbon in the phytomass of pine stands

At present, observations on permanent test plots are insufficient, and they are of a local nature. All this, of course, introduces an additional factor of uncertainty in the assessment of the organic carbon budget. The purpose of the publication is to estimate the age fluxes of organic carbon in pine stands for the 70th period using the example of a local object. The data of the current increase in carbon indicate the presence of two periods in high-density pine forests (25-42 years and 51-58 years), when the forest stands, for a short time, from the objects of “sink” of carbon become its “source”. As a result, it can be stated that timely forestry activities (thinning) will prevent the process of carbon release and promote the intensification of the “runoff” process by increasing the growth of phytomass.

Chromophoric dissolved organic carbon cycle and its molecular compositions and optical properties in precipitation in the Guanzhong basin, China

The investigation of water-soluble organic carbon (WSOC), which is important in the biogeochemical cycle of precipitation, can provide a comprehensive view of chromophores within the atmospheric boundary layer. In this work, the optical properties and molecular characteristics of WSOC in precipitation over the Guanzhong Basin (GB) of North China were investigated using ultraviolet-visible (UV–vis) absorption and excitation-emission matrix (EEM) fluorescence spectra, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with electrospray ionization (ESI). Furthermore, sources and wet deposition of WSOC were estimated using in-situ measurements and modeling. The light-absorption by WSOC at 250–300 nm (UV region) and 400–550 nm (visible region) was 64.17% and 15.36% relative to the estimated total light-absorption, respectively. Parallel factor (PARAFAC) analysis revealed three types of fluorophores in WSOC at Xi'an (XN), including two humic-like substances (HULIS) and one protein-like substance (PRLIS), with HULIS accounting for 79% of total fluorescence intensity. FT-ICR MS analysis revealed that CHO and CHON were the most abundant components of WSOC at XN, each containing a variety of lignins, protein/amino sugars, and lipids. Moreover, the positive matrix factorization (PMF) model identified the contributions from three main sources (secondary precursors and aerosols, and coal combustion) of WSOC in precipitation at XN. The annual wet deposition flux of WSOC in precipitation at XN was estimated as about 0.63 g C m−2 yr−1, lower than that at other polluted cities. These findings add to our understanding of chromophoric dissolved organic carbon budgets, which is critical for accurately assessing the global carbon cycle.

Contrasting Chemical Complexity and the Reactive Organic Carbon Budget of Indoor and Outdoor Air.

Reactive organic carbon (ROC) comprises a substantial fraction of the total atmospheric carbon budget. Emissions of ROC fuel atmospheric oxidation chemistry to produce secondary pollutants including ozone, carbon dioxide, and particulate matter. Compared to the outdoor atmosphere, the indoor organic carbon budget is comparatively understudied. We characterized indoor ROC in a test house during unoccupied, cooking, and cleaning scenarios using various online mass spectrometry and gas chromatography measurements of gaseous and particulate organics. Cooking greatly impacted indoor ROC concentrations and bulk physicochemical properties (e.g., volatility and oxidation state), while cleaning yielded relatively insubstantial changes. Additionally, cooking enhanced the reactivities of hydroxyl radicals and ozone toward indoor ROC. We observed consistently higher median ROC concentrations indoors (≥223 μg C m-3) compared to outdoors (54 μg C m-3), demonstrating that buildings can be a net source of reactive carbon to the outdoor atmosphere, following its removal by ventilation. We estimate the unoccupied test house emitted 0.7 g C day-1 from ROC to outdoors. Indoor ROC emissions may thus play an important role in air quality and secondary pollutant formation outdoors, particularly in urban and suburban areas, and indoors during the use of oxidant-generating air purifiers.

Can organic carbon and water supplementation sustain soil moisture–carbon balance under long-term plastic mulched semiarid farmland?

Plastic mulching (PM) is regarded as a promising way to increase crop production. However, its reported that plastic mulching may decrease soil organic carbon content and been unfavorable for sustainable agricultural production. Confirmation of the effects of plastic mulching on soil water and organic carbon balances in a long term, including finding efficient ways to improve these balances, is crucial for the sustainability of agricultural production in semiarid rain-fed areas. We conducted field experiment (2010–2019), with spring maize and wheat as tested crops, using four treatments of plastic mulching (PM), plastic mulching with supplementary irrigation (PMI), plastic mulching with organic fertilizer application (PMO) and without mulching (CK). The results of this 10-year field experiment showed that PM with supplementary irrigation and organic fertilizer application not only significantly increased yield and Water productivity (WP) of wheat and maize, but also had a positive effect on soil water budget. The soil water budget of PMI, PMO and PM significantly increased by 120.4%, 96.1% and 105.8% in wheat and by 149.7%, 28.2% and 53.6% in maize, compared with CK, respectively. The improved soil water condition resulted in a significant increment of yield, PMI, PMO and PM increased yield by 122.8%, 89.7% and 67.0% for wheat, by 237.8%, 183.0% and 148.4% for maize, respectively, compared with CK. The 10 years of continuous PM significantly decreased soil organic carbon content (SOC) in 0–10, 10–20, 30–50 and 50–70 cm profiles by 22.5%, 19.1%, 15.6% and 15.3% for wheat, but had no significant effect for maize. The PMO significantly increased soil organic carbon content in 0–30 cm profiles for both wheat and maize, but the soil organic carbon budgets were negative for all four treatments, the PMO accelerated soil organic carbon loss for maize but had little effect for wheat compared with PM and CK. However, PMI, PMO and PM significantly increases crop bio-carbon production, resulted in the positive total carbon budget and significantly increased by 25.0, 15.0, 11.5 Mg ha−1 in wheat and 105.1, 74.1, 74.0 Mg ha−1 in maize, respectively, as compared with CK. These results suggested that the soil organic carbon budget differed for the two crops, also affected by water or organic carbon supplementation. A more appropriate crop rotation system with organic fertilizer application should be developed, to increase crop production and soil quality under plastic mulched condition in such semiarid rain-fed areas.

Conversion of Brazilian savannah to agricultural land affects quantity and quality of labile soil organic matter

Most studies assessing land-use change (LUC) and management impacts on the soil organic carbon (SOC) budget mainly focused on total soil organic matter (SOM), neglecting its functional fractions such as water-extractable organic matter (WEOM). This may limit a better understanding of SOC dynamics along the soil profile as WEOM plays a key role in several soil and microbial processes. Here, we aimed to obtain a quantitative and qualitative assessment of WEOM dynamics to 1 m depth in areas after native vegetation conversion to long-term agriculture in Brazil. For this purpose, cropland areas under no-tillage (NT; ∼ 23 years old) and native vegetation (NV; i.e., Cerrado vegetation) were evaluated across a transect of 1000 km within Brazil’s new agricultural frontier – the so-called MATOPIBA region. We combined spectroscopic analyses (UV–Vis and DRIFT spectroscopies), C and nitrogen (N) determination in different SOM pools (i.e., bulk SOM, WEOM, and microbial biomass), and microbial measurements [e.g., microbial biomass C (MBC) and N (MBN), and β-glucosidase activity] to better understand changes in WEOM dynamics induced by LUC. We observed that although SOC decreased along the soil profile after NV (4.2–20.7 g kg−1) conversion to NT (3.8–14.2 g kg−1), water-extractable organic C levels (3.6–79.3 mg L−1) were similar between land uses. On the other hand, an increase in water-extractable total N was observed after NV (0.4–4.6 mg L−1) conversion to NT (0.5–19.3 mg L−1). Although MBC and MBN levels decreased for most study sites, β-glucosidase activity increased after LUC indicating intensive microbial processing of SOM. As a result, WEOM had lower aromaticity and molecular weight (i.e., low values of SUVA254 and A2/A3) in NT than NV; and a relative decomposition of aliphatic and polysaccharides over aromatic and amine/amide was observed after LUC through DRIFT spectroscopy. Our study indicates that LUC had stronger impacts on WEOM quality than its quantity, with agricultural areas having more biochemical changes in WEOM than NV.

Spatial configuration of seagrass community attributes in a stressed coastal lagoon, southeastern Gulf of Mexico

Terminos Lagoon is the largest coastal lagoon in the southern Gulf of Mexico and is a highly biodiverse and productive system that supports important seagrasses. These key habitats are under significant stress due to anthropic activities around and inside the lagoon. Considering the lack of updated information about their distribution, cover, and ecological condition, the objectives of this study were to assess the current morphological and seascape conditions of seagrass meadows in Terminos Lagoon and to evaluate their morphological changes in almost one decade (2010–2018). We combined remote sensing and fieldwork to assess environmental parameters (sea surface temperature and a light attenuation index), quantify seascape metrics, evaluate morphometric variables of seagrasses, and quantify organic carbon budgets. Three species of seagrasses (Thalassia testudinum, Syringodium filiforme and Halodule wrightii) were distributed in the lagoon and showed the highest densities and cover in sheltered zones, where shallow and more transparent water conditions occurred. In general, the seascape-cover diversity was regularly distributed in the study area, with the highest meadow connectivity mainly along the inner littoral region of the island. While the values we recorded for T. testudinum leaf length (X̄ = 18.6 cm), shoot densities (X̄ = 249.5 m−2) and total biomass (X̄ = 195.2 g DW m−2) were consistent with other recent studies, they were lower compared with other coastal systems in southeastern Gulf of Mexico and different between 2010 and 2018 in the lagoon, presumably associated with strong anthropic influences in the region. Finally, we provide a spatially explicit baseline information on morphology, seascape configuration and the first sensitivity map for one of the most important coastal lagoons in Mexico.

Numerical model to simulate long-term soil organic carbon and ground ice budget with permafrost and ice sheets (SOC-ICE-v1.0)

Abstract. The degradation of permafrost is a large source of uncertainty in understanding the behaviour and projecting the future impacts of Earth's climate system. The spatial distributions of soil organic carbon (SOC) and ground ice (ICE) provide essential information for the assessment and projection of risks and impacts of permafrost degradation. However, uncertainties regarding the geographical distribution and estimated range of the total amount of stored carbon and ice are still substantial. A numerical soil organic carbon–ground ice budget model, SOC-ICE-v1.0, that considers essential aspects of carbon and hydrological processes in above-ground and subsurface environments and permanently frozen ground (permafrost) and land cover changes (ice sheets and coastlines) was developed to calculate the long-term evolution of local SOC and ICE. The model was integrated to cover the last 125 kyr – from the last interglacial to date for areas north of 50∘ N at 1∘ resolution – to simulate the balance between accumulation and dissipation of SOC and ICE. Model performance was compared with observation-based data and evaluated to assess allogenic (external) impacts on soil carbon dynamics in the circum-Arctic region on a glacial–interglacial timescale. Despite the limitation of forcing climate data being constructed on the basis of a single Greenland ice core dataset, the simulated results successfully reproduced temporal changes in northern SOC and ICE, consist with current knowledge. The simulation also captured regional differences in different geographical and climatic characteristics within the circum-Arctic region. The model quantitatively demonstrated allogenic controls on soil carbon evolution represented by a key parameter that reflects climatological and topo-geographical factors. The resulting circum-Arctic set of simulated time series can be compiled to produce snapshot maps of SOC and ICE distributions for past and present assessments or future projection simulations. Examples of 1∘ resolution maps for the Last Glacial Maximum and mid-Holocene periods were provided. Despite a simple modelling framework, SOC-ICE-v1.0 provided substantial information on the temporal evolution and spatial distribution of circum-Arctic SOC and ICE. Model improvements in terms of forcing climate data, improvement of SOC and ICE dynamics, and choice of initial values are, however, required for future research.

Glacier Loss Impacts Riverine Organic Carbon Transport to the Ocean

AbstractLateral transport of organic carbon (OC) to the coastal ocean is an important component of the global carbon cycle because rivers transport, mineralize, and bury significant amounts of OC. Glaciers drive water and sediment export from many high‐elevation and high‐latitude ecosystems, yet their role in watershed OC balances is poorly understood, particularly with regard to particulate OC. Here, we evaluate seasonal water, sediment, and comprehensive OC budgets, including both dissolved and particulate forms, for three watersheds in southeast Alaska that vary in glacier coverage. We show that glacier loss will shift the dominant size fraction of riverine OC from particulate toward dissolved and potentially alter the provenance of particulate OC. Glacier coverage also controls whether OC export is source (C stock) or transport (runoff) limited at the watershed scale. These findings provide insight into the future trajectory of riverine OC export in glacierized regions.